As is well known to those skilled in the art, various aromatic fractions containing light alkylaromatics (benzene, toluene, xylene, etc.) commonly in the C.sub.6 to C.sub.12 range may be converted into other desired fractions. Such charge materials may be derived from various sources including petroleum refinery sources, coking of coal, etc. More specifically, such streams may be recovered from the so-called coal-tar by-products, from tar sands, from petroleum refinery processes, etc. Such streams are preferably treated to lower the content of nitrogen (including organic nitrogen) to less than 10 ppm, more preferably to below 1 ppm, prior to further treating.
During petroleum processing, for example, C.sub.6 to C.sub.12 fractions containing alkylaromatic hydrocarbons may be recovered as components of various streams. In the case of catalytic reforming, the reformate (after extraction of undesired fractions including aliphatics) may contain a C.sub.6 to C.sub.12 alkylaromatic hydrocarbon fraction.
A typical alkylaromatic fraction which may be obtained contains predominantly C.sub.6 to C.sub.9 hydrocarbons. Still another aromatic fraction which may be obtained, referred to as crude xylenes, actually contains hydrocarbons having 7-9 carbon atoms.
Refinery operations in which high grade gasolines are to be produced may recover an aromatic fraction containing predominantly 6-8 carbon atoms -- referred to as a BTX fraction -- containing substantial quantities of benzene, toluene, xylenes and ethylbenzene.
In processing each of these fractions, it is commonly desired to recover each of the components to permit most efficient utilization of the separated components. In the case of the crude xylenes, it may be desirable to recover substantially pure xylenes free of C.sub.7 and C.sub.9 cuts.
In the case of the C.sub.6 -C.sub.9 charge, it may be desirable to recover the toluene to permit, e.g. its use in gasolines and to thereby upgrade the latter -- in particular, to increase the back-end volatility which improves the drivability of new cars during engine warmup. More particularly, the possibility of more severe restrictions on lead anti-knock content and on the end point of motor naphtha raises a problem which may be solved by the presence of increasing proportions of toluene and C.sub.8 aromatics (xylenes and ethylbenzene) in motor fuels with decreased proportion of benzene and C.sub.9 aromatics.
Treatment of these typical streams to upgrade them may commonly include transalkylation wherein typically benzene and C.sub.9 and C.sub.10 aromatics may react to produce increased yield of desired toluene and C.sub.8 aromatics. Similarly C.sub.8 and C.sub.9 stocks can be prepared by transalkylating toluene with C.sub.10 and C.sub.11 aromatics.
Transalkylation for example may be carried out in the presence of a catalyst which may include a Group VIII metal on a solid acidic support. Typical of such solid acid catalysts may be that including:
15% -- alumina matrix PA1 85% -- 0%-12% cobalt sulfide on 40:1 SiO.sub.2 :Al.sub.2 O.sub.3 acid-leached mordenite
As alkyl transfer is carried out under predetermined conditions of temperature, pressure, flow rate, etc., it is found that the yield of desired product decreases with time. Typically, for example, when using the abovementioned acid mordenite catalyst in transalkylation at 650.degree. F., 800 psig and total LHSV (equimolor amounts of benzene and xylene) of 6, the initial toluene yield may be 40%. After 200 hours, the yield may decrease to 35%. After 500 hours, the yield may be 30%. When this latter yield is reached, it may be necessary to reactivate or regenerate the catalyst.
This undesirable decrease in catalytic activity imposes an undesirable economic burden on the operation. Although these factors are noted with respect to the preferred process of transalkylation, comparable findings are observed in the case of other alkyl transfer type processes for treating alkylaromatic compositions at aromatic-ring-maintaining conditions such as isomerization, disproportionation, etc.
It is an object of this invention to provide a process for preparing aromatic hydrocarbons. It is another object of this invention to provide a process, such as a transalkylation process, characterized by increased catalyst life. Other objects of this invention will be apparent to those skilled in the art.